In recent years, identification technology where ID (identification number) is assigned to each object so as to reveal information thereon such as history which is utilized for production management and the like has attracted attention. Above all, semiconductor devices capable of wireless data transmission/reception have developed. As such a semiconductor device, in particular, an RFID (radio frequency identification) tag (also referred to as an ID tag, an IC tag, an IC chip, an RF tag, a wireless tag, an electronic tag, a wireless chip or a transponder) and the like begin to be introduced into companies, markets, and the like.
A background art will be given using a communication system which conforms to ISO/IEC15693 which is one of RFID standards as an example. This communication system encodes data by a pulse position modulation method, which modulates a carrier wave with a frequency of 13.56 MHz at 100% or 10% and changes the position of modulation to distinguish data. An example of the case where the carrier wave is modulated at 100% is shown in FIG. 3A and an example of the case where the carrier wave is modulated at 10% is shown in FIG. 3B. A carrier wave with a modulation degree of 100% includes a state having no amplitude, while a carrier wave with a modulation degree of 10% includes a state where amplitude is changed by 10%.
The method called 4PPM (pulse position modulation) which is one of the pulse position modulation methods conforming to ISO/IE15693 is described with reference to FIG. 4A.
In FIG. 4A, a rectangle portion represents a carrier wave with a frequency of 13.56 MHz and a line between rectangles represents a modulated portion. Two-bit value “00”, “01”, “10”, and “11”; and frame codes “SOF” and “EOF” are determined by respective locations of 9.44 μs of the modulated portions in 75.52 μs of duration. Note that the duration of EOF is 37.76 μs.
In FIG. 4A, “SOF” is a signal representing the start of a frame and is sent before data is sent while “EOF” is a signal representing the end of a frame and is sent after data is sent.
A transmission-side reader/writer encodes a flag signal and data such as a command by a pulse position modulation method, modulates a carrier wave with the encoded data, and sends the modulated carrier wave to an RFID tag. A reception-side RFID tag demodulates the modulated carrier wave and reads out a pulse position to obtain data.
A common method for obtaining data on the RFID tag side is described below with reference to FIG. 4B. Note that data is sent with a carrier wave modulated at 100% by the pulse position modulation method. In an example of FIG. 4B, Two-bit value “00”, “01”, “10”, and “11” are sent as data after “SOF” which is sent as a starting signal.
Note that a reference clock signal is synchronized with the portions of a carrier wave, which is modulated at 100%. Further, a half period of the clock signal has the same length as the width of the pulse modulated at 100%. A counter which performs two-bit count with the clock signal is provided as shown in FIG. 4B, count 1 and count 2. The counter counts repeatedly from “00” to “11” while “00” indicates the first position of modulation at 100% in “SOF”. The timing where each piece of data is modulated at 100% corresponds to a counter value. Data can be obtained from the signal modulated with the pulse position modulation method in accordance with the counter value which is obtained when a carrier wave is modulated at 100%.
An RFID tag needs a reference clock signal to extract data from a carrier wave. However, the signal which can be received by RFID tag from an antenna, is only the carrier wave and a demodulated signal which is obtained by demodulating the carrier wave. Therefore, a reference clock signal for detecting a timing of modulating the carrier wave (hereinafter the timing is also referred to a pulse position) needs to be generated in the RFID tag.
A PLL (phase locked loop) circuit can be used to obtain the reference clock signal. A PLL circuit detects a phase difference between an input signal and an output signal and controls a VCO (voltage controlled oscillator) from which the output signal is generated, so that the output signal with a frequency precisely synchronized with the input signal can be obtained.
The clock signal which is used for internal operation of the RFID tag can be generated by obtaining a carrier wave or a waveform synchronized with a demodulated signal with the use of a PLL circuit. The RFID tag which generates the clock signal using the PLL circuit is disclosed, for example, in FIG. 9 of Patent Document 1 (Japanese Published Patent Application No. 2008-010849).